Magnet centering device and shield

ABSTRACT

A generally circular body of resilient material adapted to be interposed between front and rear plates of a transducer, around a pole piece centered on one of said plates, and centrally within an annular permanent magnet between the plates, to center the pole piece within the magnet and to shield a voice coil and the pole piece from foreign matter which otherwise come between them from the magnet.

United States Patent [191 Digre MAGNET CENTERING DEVICE AND SHIELD [76] Inventor: Clifford B. Digre, 4745 Vincent Ave. South, Minneapolis, Minn. 55410 22 Filed: Feb. 4, 1974 [21 Appl. No.: 439,026

[52] US. Cl. 179/119 R; 179/115.5 PC [51] Int. Cl. H04R l/OO [58] Field of Search 179/114 R, 115 R, 117,

179/119 R, 115.5 R, 115.5 BS, 115.5 ME, 115.5 PC,115.5 VC, 120; 335/231 [56] References Cited FOREIGN PATENTS OR APPLICATIONS 1,190,586 11/1957 France 179/1 15.5 PC

[ Aug.5, 1975 943,968 12/1963 United Kingdom 179/1 15.5 PC 975,983 1 H1964 United Kingdom 179/1 17 1,240,127 9/1965 Germany 179/115.5 VC

Primary ExaminerKath1een H. Claffy Assistant E.\'aminerGeorge G. Stellar Attorney, Agent, or FirmMerchant, Gould, Smith & Edell 5 7 ABSTRACT A generally circular body of resilient material adapted to be interposed between front and rear plates of a transducer, around a pole piece centered on one of said plates, and centrally within an annular permanent magnet between the plates, to center the pole piece within the magnet and to shield a voice coil and the pole piece from "foreign matter which otherwise come between them from the magnet.

2 Claims, 5 Drawing Figures MAGNET CENTERING DEVICE AND SHIELD terial of the magnet is brittle and small pieces are easily chipped or broken from the edges of the magnet. Often these small pieces become lodged between thev voice coil and the pole piece'or supporting plate, and. interfere with movement of the coil so as to seriously impair the output and tonequality of the speaker. Another problem in assembling transducers using annular magnets lies inthe accurate centering of the pole piece within the magnet and holding the pole piece centered during and after assembly of the transducer.

An important object of this invention is the provision of a shield which will adequately protect the moving voice coil and adjacent parts of the loud speaker from foreign matter such as pieces of ceramic material which may break away from the ceramic magnet of the transducer.

Another object of this invention is the provision of such a shield which accurately centers the pole piece of the transducer with respect to an annular ceramic magnet and holds it centered during and after assembly of a tran sducer.

Still another object of this invention is the provision of a shield and centering device which is simple and inexpensive to produce, and which is efficient in operation and extremely durable in use. 7

To these ends, I provide a generally cup-shaped body having a circular peripheral wall portion and a bottom wall portion, the bottom wall portion defining an axial opening therethrough for snug reception of a cylindrical pole piece projecting axially from a rear mounting plate to which is secured one side of a flat circular permanent magnet having an axial bore therethrough. The peripheral wall portion of the body has an outer circular surface for snug reception in the bore of the magnet when the bottom portion of the body is in abutting engagement with the rear mounting'plate. The body is made from resilient material and has an axial dimension substantially equal to the axial thickness of the magnet so that, when a front mounting "plate is=secured to the magnet opposite the rear mounting plate, the marginal edge of the body will abut the front mounting plate "and the bottom surface thereof will abut the rear mounting plate, to effectively shield the'interior of the r body and parts contained therein from entry of foreign matter.

BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In the drawings, a loud speaker is indicated in its entirety by the numeral 1, the same including a generally .frustoconical frame 2, that supports the marginal edge of a relatively larger diameter cone 3, a mounting gasket 4, a relatively smaller diameter cone 5, a spider 6, a voice coil 7, an annular permanent magnet 8, front and rear mounting plates 9 and 10 for the magnet 8, and a pole piece 11 projecting axially forwardly from the rear mounting plate 10. r I

The voice coil 7 is wound on a tubular bobbin 12 that is of a size to fit freely over the pole piece 11 and to have a clearance fit with an axial opening 13 in the front plate 9. The bobbin 12 extends freely through an axial opening 14 in the flat base portion 15 of the frame 2. In the type of speakerillustrated, a pair of mounting holes 16 inv the base portion 15 are alignable with a pair of threaded openings 17 in the front plate 9 for reception of anchoring screws, not shown, for rigidly securing the frame 2 to the front plate 9.

The spider 6 has its marginal edge 18 cemented or otherwise rigidlysecured to the base portion 15 of the frame 2 and is provided with a central opening 19 for reception of the front end of the bobbin 12. The cone 3 has an axial opening 20 aligned with the opening 19 and the spider 6, the front end of the bobbin 12 being rigidly secured in the aligned openings 19 and 20 and to the rear or inner end of the cone 5 in the usual manner, by suitable adhesives, not shown. The outer marginal edge of the low frequency cone 3 is likewise cemented to the frame 2.

The magnet 8 is of the type commonly known as a ceramic magnet and is annular in form, having a flat front and rear sides 21 and 22 respectively and a cylindrical bore 23. In assembly, the magnet 8 is disposed in faceto-face engagement of its flat sides 21 and 22 with the front and rear mounting plates 9 and 10 respectively. The mounting plates 9 and 10, together with the pole piece 11, are made from iron or similar paramagnetic -front mounting plate 9 and cooperates therewith to define an annular-passage 24 for reception of the voice coil 7 and bobbin 12.

As thus far described, the speaker I is conventional in design and well known to those familiar with the transducer art. Permanent magnets, such as ceramic magnets of the type above described, have a definite advantage of providing excellent magnetic qualities 'while occupying a rather small space in a direction axially of the speaker. This enables transducers, such as speakers, to be produced which occupy a minimum of axial depth while producing very satisfactory audio output. However, one disadvantage of using a ceramic magnet in a transducer, lies in the fact that the material of the magnet is quite brittle, and often small particles thereof break or chip off of the corner edges thereof. It has been found that in many cases, such particles break off from the bore portion of the magnet and land upon the voice coil 7 or bobbin 12, being carried into the space therebetween and the opening 13 or the pole piece 11. The particles become lodged between the movable voice coil 7 and one of the stationary parts 9 or 11 and interfere with the free movement of the voice coil 7 with its bobbin 12. Thus, the output of the speaker becomes seriously impaired, or the speaker fails altogether.

To shield the voice coil 7 with its bobbin l2, and the pole piece 11, from having fragments of the magnet 8 from coming into engagement therewith, I provide a generally cup-shaped body 25 which serves the dual function of protecting the voice coil and pole piece, and centering the magnet 8 with respect to the pole piece 11 during assembly of the magnet structure. The body 25 is formed to provide a circular wall portion 26 and a generally flat bottom wall portion 27, these portions being connected by a curved portion 28. As shown in FIGS. 3 and 4, the wall structure of the body 25 is uniform in thickness, the cylindrical wall portion 26 having an outer, generally cylindrical or circular wall surface 29 that snugly fits within the bore 23 of the ceramic permanent magnet 8. The bottom wall portion 27 is formed to provide an axial opening 30 that snugly slidably engages the pole piece 11, and the-cylindrical wall portion 26 has a marginal edge 31 which, when the body 25 is placed on the pole piece 11 with the bottom wall 27 engaging the rear mounting plate 10, is adapted to abuttingly engage the front mounting plate 9. The axial dimension of the body 25 between the outer surface of the bottom wall 27 thereof and the marginal edge 31 is substantially equal to the axial thickness of the permanent magnet 8 so that, when the mounting plates 9 and are adhered to the magnet 8, the body snugly fits between the plates 9 and 10. The body 25 is made from resilient material, preferably a synthetic plastic material, such as nylon, polyethylene or the like.

The modified form of shield shown in FIG. 5 is indicated generally at 32, having a cylindrical wall 33 and an axially inner end or bottom wall portion 34 that defines an axial pole piece receiving opening 35. The shield 32 operates in the same manner as the shield or body 25.

In assembling the magnet structure of FIG. 4, the body 25 is first slipped over the pole piece 11 and moved axially thereof until the bottom wall portion 27 is in abutting relationship with the end plate 10. Adhesive is then applied to the surface 22 of the magnet 8, or to the front surface of the rear plate 10, or both, and the magnet 8 moved telescopically over the body 25 into face-to-face engagement with the rear mounting plate 10. In view of the fact that the outer peripheral wall surface 29 of the body 25 closely engages the inner bore surface 23 of the magnet 8, and that the axial opening has close fitting relationship with the pole piece 11, the body 25 accurately centers the pole piece 11 with respect to the magnet 8. The front mounting plate 9 is then adhered to the surface 21 of the magnet 8 in concentric relationship to the pole piece 11. For the purpose of obtaining a true concentric relationship between the pole piece 11 and mounting plate 9, a suitable piece of tubular material, not shown, may be slipped over the pole piece 11 and used as a jig for accurately positioning the front mounting plate 9. After the adhesive has set, the jig may be removed from the assembled magnet structure.

In the event that manufacturing tolerances result in the axial length of the body 25 being slightly greater than the axial distance between the mounting plates 8 and 9, the resiliency of the material from which the body 25 is produced, and the curved portion 28, will permit the body 25 to yield so that the mounting plate 9 may be pressed against the surface 21 of the magnet 8. In assembly, the mounting plates 9 and 10 are clamped against their respective surfaces of the magnet 8 until the adhesive has set. However, with some contact adhesive, clamping is not necessary. It will be appreciated that the body 25 or shield 32 may be used with equal effectiveness in transducers wherein the magnets and plates are secured together by screws, rivets, or any suitable joining means.

From the above, it will be seen that the body 25 performs a dual function, that of accurately centering the magnet 8 with respect to the pole piece 11, and thereafter effectively shielding the pole piece 11 and voice coil 7 with its bobbin 12 against any fragments of material which might break away from the magnet 8.

While I have shown and described a commercial embodiment of shield and magnet centering device, and a modified form thereof, it will be understood that the same is capable of further modification without departure from the spirit and scope of the invention, as defined in the claims.

What is claimed is:

l. A shield and magnet centering device for a transducer of the type including an annular permanent magnet having axially opposite flat surfaces and an axial bore therethrough, and front and rear plates of paramagnetic material joined to said axially opposite surfaces of the magnet, said rear plate having a pole piece projecting axially through said bore, said front plate having an axial opening therethrough for reception of said pole piece; said shield and centering device comprising a generally cup-shaped body of nonmagnetic resilient material having a peripheral wall portion defining a circular outer surface portion for snug engagement with the axial bore through the magnet and a bottom wall portion having an axially outer surface for abutting engagement with said rear plate, said bottom wall portion having an axial opening therethrough for snug reception of said pole piece, whereby to center the pole piece within said bore during the joining of said rear plate to said magnet, said body having an axial tions. 

1. A shield and magnet centering device for a transducer of the type including an annular permanent magnet having axially opposite flat surfaces and an axial bore therethrough, and front and rear plates of paramagnetic material joined to said axially opposite surfaces of the magnet, said rear plate having a pole piece projecting axially through said bore, said front plate having an axial opening therethrough for reception of said pole piece; said shield and centering device comprising a generally cup-shaped body of nonmagnetic resilient material having a peripheral wall portion defining a circular outer surface portion for snug engagement with the axial bore through the magnet and a bottom wall portion having an axially outer surface for abutting engagement with said rear plate, said bottom wall portion having an axial opening therethrough for snug reception of said pole piece, whereby to center the pole piece within said bore during the joining of said rear plate to said magnet, said body having an axial dimension substantially equal to that of said annular magnet and a marginal edge opposite said bottom wall portion so that, when said magnet and plates are disposed in face-to-face engagement, the marginal edge and said axially outer surface of said shield will have axially abutting engagement with said front and rear plates respectively.
 2. The shield and magnet centering device defined in claim 1 in which said body has a substantially uniform wall thickness and includes a cross-sectionally curved portion between said bottom and peripheral wall portions. 